Strings

Strings as a List of Letters

Perform the following functions in the console.
We can think of strings as a list of letters:

If we want to print out each letter individually, we can use a loop:

test_string = "FALL2018"
# Print each letter individually
for letter in test_string:
  print(letter)

If we only want the first letter, we can extract it using an index:

# Select only the first letter
print(test_string[0])

Or maybe we want to remove the 4 digits on the end, keeping only the first 4. We can do this using slicing, which is indicated using a colon “:”:

# Keep the first 4 letters
print(test_string[:4])

Oops, or maybe we want to keep the last 4 instead of removing them:

# Keep the last 4 letters
print(test_string[-4:])

What if we wanted to print EVERY OTHER letter? We can do this by passing an optional third parameter to the slice using a second colon:

# Print every other letter
print(test_string[::2])

Try playing around with other step sizes. Negative values are allowed!

A note about starting at 0

Python indexing feels really weird when you first encounter it. Why is the first item indexed with zero? Why is the last item in the slice not included?

It turns out this leads to some cool properties:

• The len(s[a:b]) is just b-a
• Concatenation: s[:b] + s[b:] gives us back s
• Negative indexing: len(s[-b:]) is b

Begin by downloading the short answer starter file (click this link), and save it locally. Open the file with either Thonny or a text editor. Answer the following short answer question (1-2 English sentences.):
SA Question #1: What are two applications where you would use indexing to get part of a string?

.lower(), .upper(), and .capitalize()

The first three methods we’ll explore modify the case of a string (i.e. whether it is written with lowercase or uppercase letters).

To see this in action, try running the following commands in the console:

s = "aBcDeFg"
print("UPPERCASE:", s.upper())
print("lowercase:", s.lower())
print("Capitalize:", s.capitalize())

.split()

The .split() method is useful for breaking a string into pieces. To see how this works, try the following:

sentence = """It was a bright cold day in April, 
and the clocks were striking thirteen."""
print("Individual words:", sentence.split())

Because .split() gives us back a list (see the square brackets?), we can use the results of a call to .split() inside a loop. Note: We haven’t learned about loops yet, so we don’t expect you to fully understand the following code or use them in your lab/homework, but see if you get the general idea.

for word in sentence.split():
   print(word)

Calling .split() without anything inside the parentheses tells Python that we want it to split the string on whitespace (spaces, tabs, etc.). However, we can choose to split on any delimiter we want by passing it in as input to the .split() method:

print("Before and after the comma:", sentence.split(','))

(Notice the newline character \n from where we hit return in the multiline string!)

.join()

You can think of the .join() method as the undo of the .split() method: it uses copies of whatever string it is called on to concatenate the strings that are passed into it. For example:

words = sentence.split()
with_dashes = '-'.join(words)
print(with_dashes)

.replace()

The .replace() method makes it really easy to (as its name implies) replace instances of one substring with another. For example, we could change all appearances of the letter 'a' to 'oo':

phrase = "I like to eat apples and bananas"
print(phrase.replace('a', 'oo'))

Answer the following short answer question (1-2 English sentences) in your short answer question file for this lab:
SA Question #2: What is a real world example where you might use .split(), .join(), or .replace()?